Magnetorheological Fluids
In the 1950s, it was discovered that fluids could be created whose resistance to flow were modifiable by subjecting them to a magnetic or electric field. This was disclosed in U.S. Pat. No. 2,661,596, which is hereby incorporated by reference, where the inventor also disclosed its use in a hydraulic device. Those fluids that are responsive to an electrical field are known as electrorheological fluids while those responsive to magnetic fields are magnetorheological. Of these two, magnetorheological fluids have been the easier to work with, as their electrical counterparts are susceptible to performance-degrading contamination and require strong electric fields, which necessitate complicated, expensive high-voltage power supplies and complex control systems. In contrast, both permanent magnets and electromagnets are inexpensive and easy to produce, while the magnetorheological fluids are not as sensitive to contamination.
Magnetorheological (MR) fluids can be formed by combining a low viscosity fluid, such as a type of oil, with magnetic particles to form a slurry. The original patent used particles of iron on the order of 0.1 to 5 microns, with the particles comprising 20% or more by volume of the fluid. More recent work in MR fluids can be found, for instance, in U.S. Pat. No. 6,280,658. When a magnetic field passes through the fluid, the magnetic particles align with the field, limiting movement of the liquid due to the arrangement of the iron particles. As the field increases, the MR fluid becomes increasingly solid, but when the field is removed, the fluid resumes its liquid state again. FIG. 1 is a graph of the flow rate of an exemplary MR fluid through 0.4 inch inner diameter tubing versus the strength of the magnetic field applied to the fluid. In each case, the flow rate goes to zero as the field increases. Magnetorheological fluids have been used in such areas as dampers, locks, brakes, and abrasive finishing and polishing, with over 100 patents issued that utilize these fluids. MR fluids can be obtained from the Lord Corporation of Cary, N.C.
Downhole Equipment
Devices that are used in the development and production of hydrocarbon wells have a number of constraints to which they must adhere. They must be capable of handling the harsh environment to which they are subjected, be controllable from the surface, and be sized to fit within the small area of a borehole, yet the fact that they can be operating thousands of feet underground makes their reliability a high priority. Some of the problems encountered in drilling and production of hydrocarbons are as follows:
1) It is imperative to reliably be able to trigger an event when desired, but not before. For instance, the firing of guns used to create openings through the casing into a formation must release enough energy to fracture through not only the casing, but also through damaged sections of the formation. Premature firing of the guns is both a safety issue (i.e., personnel can be injured) and an economic issue (equipment can be damaged, openings made into undesired strata must be repaired or bypassed).
2) Many pieces of equipment used downhole have valves that must be opened and closed. In other equipment, the relationship between two parts must be fixed at some points in time, yet moveable at others, such in a travel joint, which makes up for the movement of a drilling ship as it floats on the surface of the ocean. Traditional apparatus has relied various physical means to operate valves or release a part from a fixed relationship. These can include rotating the drill string to release a J-fastener, relying on pressure, either within the string or in the annulus, to rupture a valve or to apply the pressure necessary to move a part, and shear pins or similar devices. It is desirable to have more reliable means of operating this equipment more precisely. Additionally, the use of moving parts leads to rigorous designs that have redress costs and require rig time to trigger the valves. It would be desirable to utilize solid-state valves to lower costs, improve reliability, and decrease rig time for activation.
3) It would be desirable to provide a simple means for performing logical control steps, without the use of moving parts.
4) Devices such as packers traditionally use hard rubber parts to seal between the downhole tubing and the casing or borehole. The rubber requires high pressures to set, and the inflatable packers that have been used will not hold the large differential pressures of those using rubber packers. An alternative is desirable that would not require large amount of force to set, but that would handle large differential pressures.
Because of the variety of devices disclosed in the current application, specific examples of prior art devices are more fully discussed before the inventive alternative is disclosed.